549 research outputs found
HST-NICMOS Observations of M31's Metal Rich Globular Clusters and Their Surrounding Fields: I. Techniques
We have obtained HST-NICMOS observations of five of M31's most metal rich
globular clusters. These data allow photometry of individual stars in the
clusters and their surrounding fields. However, to achieve our goals -- obtain
accurate luminosity functions to compare with their Galactic counterparts,
determine metallicities from the slope of the giant branch, identify long
period variables, and estimate ages from the AGB tip luminosity, we must be
able to disentangle the true properties of the population from the
observational effects associated with measurements made in very crowded fields.
In this paper we present a careful analysis of photometry in crowded regions,
and show how image blending affects the results and interpretation of such
data. Although this analysis is specifically for our NICMOS observations in
M31, the techniques we develop can be applied to any imaging data taken in
crowded fields; we show how the effects of image blending will even limit NGST.
We use three different techniques to analyze the effects of crowding on our
data, including the insertion of artificial stars (traditional completeness
tests) and the creation of completely artificial clusters. They are used to
derive threshold- and critical-blending radii for each cluster, which determine
how close to the cluster center reliable photometry can be achieved. The
simulations also allow us to quantify and correct for the effects of blending
on the slope and width of the RGB at different surface brightness levels.Comment: AAS LaTeX v5.0, 18 pages. Submitted to the A
WFPC2 Observations of Massive and Compact Young Star Clusters in M31
We present color magnitude diagrams of four blue massive and compact star
clusters in M31: G38, G44, G94, and G293. The diagrams of the four clusters
reveal a well-populated upper main sequence and various numbers of supergiants.
The U-B and B-V colors of the upper main sequence stars are used to determine
reddening estimates of the different lines of sight in the M31 disk. Reddening
values range from E(B-V) = 0.20 +/- 0.10 to 0.31 +/- 0.11. We statistically
remove field stars on the basis of completeness, magnitude and color. Isochrone
fits to the field-subtracted, reddening-corrected diagrams yield age estimates
ranging from 63 +/- 15 Myr to 160 +/- 60 Myr. Implications for the recent
evolution of the disk near NGC 206 are discussed.Comment: 17 pages, Latex, ApJ, in Pres
Subaru/HDS Abundances in Three Giant Stars in the Ursa Minor Dwarf Spheroidal Galaxy
With the HDS (High Dispersion Spectrograph) on the Subaru telescope, we
obtained high resolution optical region spectra of three red giant stars (cos
4, cos 82, and cos 347) in the Ursa Minor dwarf spheriodal galaxy. Chemical
abundances in these stars have been analysed for 26 elements including alpha-,
iron-peak, and neutron capture elements. All three stars show low abundances of
alpha-elements (Mg, Si, and Ca) and two stars (cos 82 and cos 347) show high
abundance of Mn compared to Galactic halo stars of similar metallicity. One
star (cos 4) has been confirmed to be very metal deficient ([Fe/H]=-2.7) and
found to show anomalously low abundances of Mn, Cu, and Ba. In another star cos
82 ([Fe/H]=-1.5), we have found large excess of heavy neutron-capture elements
with the general abundance pattern similar to the scaled solar system r-process
abundance curve. These observational results are rather puzzling: low
abundances of alpha-elements and high abundance of Mn seem to sugggest a
significant contribution of SNe Ia at low metallicity, while there is no hint
of s-process (i.e., AGB stars) contribution even at [Fe/H]=-1.5, suggesting a
peculiar nucleosynthetic history of the UMi dSph galaxy.Comment: 26 pages with 12 figures, accepted to PAS
DeficiĂȘncias minerais em pastagens do Estado do Rio de Janeiro.
bitstream/item/65349/1/CTAA-DOCUMENTOS-8-DEFICIENCIAS-MINERAIS-EM-PASTAGENS-DO-ESTADO-DO-RIO-DE-JANEIRO-FL-02171.pd
HST-NICMOS Observations of M31's Metal Rich Globular Clusters and Their Surrounding Fields: II. Results
We have obtained HST-NICMOS observations of five of M31's most metal rich
globular clusters: G1, G170, G174, G177 & G280. For the two clusters farthest
from the nucleus we statistically subtract the field population and estimate
metallicities using K-(J-K) color-magnitude diagrams (CMDs). Based on the
slopes of their infrared giant branches we estimate [Fe/H]=-1.22+/-0.43 for G1
and -0.15+/-0.37 for G280. We combine our infrared observations of G1 with two
epochs of optical HST-WFPC2 V-band data and identify at least one LPV based on
color and variability. The location of G1's giant branch in the K-(V-K) CMD is
very similar to that of M107, indicating a higher metallicity than our purely
infrared CMD: [Fe/H]=-0.9+/-0.2.
For the field surrounding G280, we estimate the metallicity to be -1.3 with a
spread of 0.5 from the slope and width of the giant branch. Based on the
numbers and luminosities of the brightest giants, we conclude that only a small
fraction of the stars in this field could be as young as 2 Gyr, while the
majority have ages closer to 10 Gyr.
The K-band luminosity functions (LFs) of the upper few magnitudes of G1 and
G280, as well as for the fields surrounding all clusters, are indistinguishable
from the LF measured in the bulge of our Galaxy. This indicates that these
clusters are very similar to Galactic clusters, and at least in the surrounding
fields observed, there are no significant populations of young luminous stars.Comment: AAS LaTeX v5.0, 17 pages. Submitted to the A
The Globular Cluster System of NGC 5128 II. Ages, Metallicities, Kinematics, and Formation
We present a study of the nearby post-merger giant elliptical galaxy, NGC
5128 (Centaurus A), in which we use the properties of its globular cluster (GC)
and planetary nebula (PN) systems to constrain its evolution. Using photometric
and spectroscopic data for 215 GCs presented in Paper I, we study trends in
age, metallicity, and kinematics for the GC system. We confirm that the GC
metallicity distribution is bimodal, and show that these two sub-populations
have different properties. Using spectral line index measurements of the
brightest clusters, the metal-poor GCs have old ages like the Milky Way
globular clusters, while the metal-rich GCs have H-beta line-strengths that
could be interpreted as a mean age of ~5 (+3/-2) Gyr. Both populations appear
to have [Mg/Fe] ratios consistent with that of the Galactic GC system, although
this quantity is not very well-constrained. The kinematics of the metal-rich
GCs are similar to those of the planetary nebulae, exhibiting significant
rotation about a misaligned axis, while the metal-poor GCs have a higher
velocity dispersion and show a weaker kinematic correlation with the field
stars. The total gravitating mass of NGC 5128 derived from the GCs is in
excellent agreement with the value derived from stellar (PN) kinematics. We
suggest that these and other data support a picture in which the main body of
NGC 5128 was formed 3-8 Gyr ago by the dissipational merger of two unequal-mass
disk galaxies supplemented by the continual accretion of both gas-rich and
gas-poor satellites.Comment: 15 pages, 21 figures (figures 14-20 best viewed in color), accepted
for publication in the Astrophysical Journa
Spectroscopic evolution of dusty starburst galaxies
By using a one-zone chemical and spectrophotometric evolution model of a disk
galaxy undergoing a dusty starburst, we investigate, numerically, the optical
spectroscopic properties in order to explore galaxy evolution in distant
clusters. We adopt an assumption that the degree of dust extinction
(represented by ) depends on the ages of starburst populations in such a
way that younger stars have larger (originally referred to as selective
dust extinction by Poggianti & Wu 2000). In particular, we investigate how the
time evolution of the equivalent widths of [OII]3727 and H is
controlled by the adopted age dependence. This leads to three main results: (1)
If a young stellar population (with the age of yr) is more
heavily obscured by dust than an old one ( yr), the galaxy can show
an ``e(a)'' spectrum characterized by strong H absorption and
relatively modest [OII] emission. (2) A dusty starburst galaxy with an e(a)
spectrum can evolve into a poststarburst galaxy with an a+k (or k+a) spectrum
0.2 Gyr after the starburst and then into a passive one with a k-type spectrum
1 Gyr after the starburst. This result clearly demonstrates an evolutionary
link between galaxies with different spectral classes (i.e., e(b), e(a), a+k,
k+a, and k). (3) A dusty starburst galaxy can show an a+k or k+a spectrum even
in the dusty starburst phase if the age-dependence of dust extinction is rather
weak, i.e., if young starburst populations with different ages (
yr) are uniformly obscured by dust.Comment: 27 pages 12 figures,2001,ApJ,in pres
Chemical Composition of Extremely Metal-Poor Stars in the Sextans Dwarf Spheroidal Galaxy
Chemical abundances of six extremely metal-poor ([Fe/H]<-2.5) stars in the
Sextans dwarf spheroidal galaxy are determined based on high resolution
spectroscopy (R=40,000) with the Subaru Telescope High Dispersion Spectrograph.
(1) The Fe abundances derived from the high resolution spectra are in good
agreement with the metallicity estimated from the Ca triplet lines in low
resolution spectra. The lack of stars with [Fe/H]=<-3 in Sextans, found by
previous estimates from the Ca triplet, is confirmed by our measurements,
although we note that high resolution spectroscopy for a larger sample of stars
will be necessary to estimate the true fraction of stars with such low
metallicity. (2) While one object shows an overabundance of Mg (similar to
Galactic halo stars), the Mg/Fe ratios of the remaining five stars are similar
to the solar value. This is the first time that low Mg/Fe ratios at such low
metallicities have been found in a dwarf spheroidal galaxy. No evidence for
over-abundances of Ca and Ti are found in these five stars, though the
measurements for these elements are less certain. Possible mechanisms to
produce low Mg/Fe ratios, with respect to that of Galactic halo stars, are
discussed. (3) Ba is under-abundant in four objects, while the remaining two
stars exhibit large and moderate excesses of this element. The abundance
distribution of Ba in this galaxy is similar to that in the Galactic halo,
indicating that the enrichment of heavy elements, probably by the r-process,
started at metallicities [Fe/H] < -2.5, as found in the Galactic halo.Comment: 15 pages, 6 figures, 6 tables, A&A, in pres
Preprocessing Among the Infalling Galaxy Population of EDisCS Clusters
We present results from a low-resolution spectroscopic survey for 21 galaxy
clusters at selected from the ESO Distant Cluster Survey. We
measured spectra using the low-dispersion prism in IMACS on the Magellan Baade
telescope and calculate redshifts with an accuracy of . We
find 1763 galaxies that are brighter than in the large-scale cluster
environs. We identify the galaxies expected to be accreted by the clusters as
they evolve to using spherical infall models and find that
to of the cluster population lies outside the virial radius
at . For analogous clusters at , we calculate that the ratio
of galaxies that have fallen into the clusters since to those that
were already in the core at that redshift is typically between and
. This wide range of ratios is due to intrinsic scatter and is not a
function of velocity dispersion, so a variety of infall histories is to be
expected for clusters with current velocity dispersions of km s. Within the infall regions of clusters, we find a larger red fraction of galaxies than in the field and
greater clustering among red galaxies than blue. We interpret these findings as
evidence of "preprocessing", where galaxies in denser local environments have
their star formation rates affected prior to their aggregation into massive
clusters, although the possibility of backsplash galaxies complicates the
interpretation.Comment: Accepted for publication in Ap
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